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Reading out of write-combining mapped memory is typically very slow since the CPU doesn't prefetch. However some archs have special instructions to do this. So add a best-effort memcpy_from_wc taking dma-buf-map pointer arguments that attempts to use a fast prefetching memcpy and otherwise falls back to ordinary memcopies, taking the iomem tagging into account. The code is largely copied from i915_memcpy_from_wc. Cc: Daniel Vetter <daniel@ffwll.ch> Cc: Christian König <christian.koenig@amd.com> Suggested-by: Daniel Vetter <daniel@ffwll.ch> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com> Acked-by: Christian König <christian.koenig@amd.com> Acked-by: Daniel Vetter <daniel@ffwll.ch> Link: https://lore.kernel.org/r/20210602083818.241793-5-thomas.hellstrom@linux.intel.com Link: https://patchwork.freedesktop.org/patch/msgid/20210602083818.241793-5-thomas.hellstrom@linux.intel.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.